Which of these best defines an asteroid?

A small celestial body that orbits the sun accurately defines an asteroid. Asteroids are primarily found in the asteroid belt, which is located between the orbits of Mars and Jupiter. Unlike planets, asteroids are generally smaller in size and have irregular shapes. They are remnants from the early solar system, composed mostly of rock and metal. This definition captures the essential characteristics and behavior of asteroids, distinguishing them from other celestial bodies such as comets or planets.

The other options, while related to the broader field of astronomy, do not precisely encapsulate the nature of asteroids. For example, referring to asteroids as a type of comet or linking them to being fragments of a planet can be misleading, as these concepts pertain to different classes of objects with distinct features and evolutionary histories. Similarly, the comparison to terrestrial planets does not apply, as asteroids do not meet the criteria of being large, terrestrial bodies. Thus, the definition provided in the correct answer effectively covers the fundamental aspects that classify an object as an asteroid.